The present invention relates to an X-ray cable, more detailed an electric cable for supplying a X-ray tube. More particularly it relates to an X-ray conduit which has an inner conductor or a cable core with several inner conductors, an inner conducting sleeve, a high voltage insulation, an outer conducting sleeve, a screen and a casing. Its purpose is to make the transient over-voltages eventually carried during the X-ray operation as not damaging.
The above specified construction of the X-ray conductor is known in the art and is disclosed for example in the German Patent 972,701. As specified hereinabove, it includes, in addition to the high voltage conductor, the inner conducting layer formed as a conducting sleeve, a high voltage insulation, and an outer conducting layer, and, a screen arranged over them and formed for example as a concentric outer cable, and finally a casing. In the course of time the cable core or the inner conductor arrangement have been subjected to different developments, while to the contrary the remaining cable structure remains the same. The conventional conductor arrangements which are used now usually include the following. In the core of the X-ray cable, in addition to the bare high voltage conductor there are two insulated heating conductors, while the round, fine- wire high voltage conductor is subdivided for symmetry into two round, half conductors so that in the cable core there are four elements stranded with one another (F&G Prospectus "Elektrotechnik" 12.72, page 23).
In the core of the X-ray cable the both insulated heating conductors are stranded with an isolated grid-driving conductor, a conductive sheathing is located on it, and then the concentric high voltage conductor is stranded (DE-GM 8,526,448).
The concentric construction is provided in that the heating conductor 1, the insulation, the heating conductor 2, the insulation, the high voltage conductor are concentrically braided (F&G Prospectus, "X-Ray Conductors" 04.89). In all cases the following construction is accepted: the inner conducting layer, the high voltage insulation, the outer conducting layer, the screen and the casing.
For the inner conductor, a strand of thin, zinced copper wires is used, which can be reinforced in its core by zinc steel wires for pulling resistance. For the conducting sleeves, semiconductor rubber or synthetic plastic mixtures (compounds) bands or foils are used. For the high voltage, cross-linked rubber or synthetic plastic mixtures are used, such as elastomers, for example EPR. For the outer conductor, a strand or a braid of copper wires is used. For the casing, rubber or synthetic plastic mixture such as PVC or glass yarn braid are used.
During the X-ray operation electrical unloading or short-circuiting can occur in the X-ray tubes. As a result, transient over voltages or wandering waves occur and are withdrawn through the X-ray conduit. This high frequency over voltages can lead to damages and disruption of electronic devices and structural elements located close to the disturbance source, such as X-ray tube or cable. For avoiding such disturbances, it is known to electrically screen the disturbance source and to reduce or suppress the propagation of the transient over voltages through the X-ray conduit by damping members.
As for the screening, the German document DE-A-1,540,332 discloses a cable sheathing for screening electromagnetic disturbance signals. Here between the cable cord and the casing, there are two braids with wires in one braid composed of pure iron and wires in the other braid composed of iron-nickel alloy with relatively high permeability. The first mentioned braid faces the respective incoming or outgoing disturbance source. The purpose of such a screening is to suppress disturbance signals for the whole electromagnetic spectrum from direct current to the microwave frequencies. As for the cable, it is always stated that it is composed of one group of wires or cables, the screening provides a damping of disturbance signals in a transverse but not in a longitudinal direction of the cable, and the problems of the transient over voltages occuring in the X-ray conduits still are not eliminated.
For solving these problems in the X-ray cable, sensing members switched in the conductor circuit are proposed in different shapes and arrangements. For example the German document DE-A-2,010,143 discloses high voltage cable for an X-ray tube in which a damping resistance is vulcanized in the high voltage plug, which connects the cable with the tube. The resistance can be formed as ohmic resistance (resistance wire), an inductive resistance (conductor coil on a core of greater magnetic permeability), or a combination of both. The German document DE-A1-3,929,402 discloses an X-ray device in which a high frequency-operating damping impedance is arranged in the high voltage cable or in the output of the high voltage generator. In the first arrangement it is composed of a ferrite core which surrounds the cable as a hollow cylinder while in the second arrangement it is composed of a resistance (diode or condensor) which is connected in parallel to the output of the generator. On the one hand this approach requires an additional damping member with significant expenses, and on the other hand its efficiency is to be improved.